1. Field of the Invention
The present invention relates to a method of marking a sintered body with ID information and more particularly, relates to a method of marking a sintered body with ID information having high contrast while minimizing contamination. The present invention also relates to a method for manufacturing a wafer for magnetic heads including performing a marking process and further relates to a sintered body that has been marked with ID information, a magnetic head, and a storage medium drive.
2. Description of the Related Art
Recently, a thin-film magnetic head having any of various structures often includes a magnetic head slider for use in a hard disk drive (HDD), a tape storage and a flexible (or floppy) disk drive (FDD), for example. Examples of wafers for such a thin-film magnetic head include sintered wafers having compositions such as Al2O3—TiC, SiC and ZrO2.
FIG. 1A illustrates a typical thin-film magnetic head slider 10. On its tracking side, this magnetic head slider 10 includes two side rails 11 that are arranged to be opposed to the surface of a magnetic disk. The surface of the thin-film magnetic head slider 10 on which the side rails 11 are provided is sometimes called an “air bearing surface (ABS)”. If the magnetic disk is rotated at a high velocity by a motor, for example, while the surface of the magnetic disk is pressed lightly by the side rails 11 of the magnetic head slider 10 by way of a head suspension, then an air layer will be formed on the surface of the magnetic disk and will reach the back surface of the air bearing surface of the slider 10. As a result, the magnetic head slider 10 is slightly lifted up. In this manner, the magnetic head slider 10 can perform read and write operations on the magnetic disk while “flying” near the surface of the disk so to speak.
A thin film 12, which causes a magnetic interaction with a storage medium such as a magnetic disk, is deposited on one end surface of the magnetic head slider 10. The thin film 12 is used to form part of an electrical/magnetic transducer. To indicate the type of the product, an identifier (ID or ID mark) 13 such as a serial number is inscribed on the other end surface of the magnetic head slider 10. Methods of inscribing an identifier 13 on sintered wafers are disclosed in Japanese Laid-Open Publications Nos. 9-81922, 10-134317 and 11-126311, for example.
In a typical manufacturing process, the magnetic head slider 10 is obtained by cutting out a bar 20 shown in FIG. 1B from a sintered wafer 1 shown in FIG. 1C and then dicing the bar 20 into a great number of chips. In FIG. 1C, the end surface 4 of the sintered wafer 1 is parallel to the air bearing surface of the magnetic head slider 10 shown in FIG. 1A.
Recently, as the sizes of such a thin-film magnetic head have been decreased to reduce the sizes and weight of an electronic appliance, the thickness of the sintered wafer 1 (corresponding to the length L of the magnetic head slider 10) and the thickness T of each bar 20 (corresponding to the height of the magnetic head slider 10) have also been reduced. For example, a magnetic head slider, which is called a “pico-slider”, has a length L of about 1.2 mm and a thickness T of about 0.3 mm. As for a magnetic head slider of such drastically reduced sizes, the sizes of characters to be inscribed on the slider should also be reduced correspondingly.
In the prior art, a laser marking method is often used to inscribe the identifier 13 (this process step will be sometimes referred to herein as a “marking” process step or an “ID marking” process step). In the laser marking method, the identifiers 13 shown in FIGS. 1A and 1B are marked on the back surface 3 of the wafer 1 that is yet to be divided into the bars 20. After the marking process step is finished, various thin films 12 are stacked on the front surface 2 of the wafer 1.
Hereinafter, the conventional laser marking method will be described briefly with reference to FIG. 2.
In the laser marking method, the back surface 3 of the sintered wafer 1 is locally irradiated with a laser beam 6 that has been converged by a lens 5, thereby rapidly heating and vaporizing the irradiated portion of the wafer 1. In this case, a tiny concave portion is formed on the back surface 3 of the wafer 1, while the material of the sintered wafer 1 is scattered around and just a portion of the scattered material is deposited on the wafer 1 again. By scanning the back surface 3 of the wafer 1 with the laser beam 6, the concave portions can be arranged so as to form an arbitrary pattern on the back surface 3 (which will be referred to herein as a “concave pattern”). Any of various types of identifiers 13 can be written at an arbitrary location on the wafer 1 by forming a concave pattern, which is made up of alphanumeric and/or numeric characters or a barcode, on the back surface 3 of the wafer 1.
A laser marking method as described above, however, has the following drawbacks.
Firstly, the portion of the sintered material that has been scattered around as a result of the exposure to the laser beam is likely adsorbed or deposited as dust onto the inscribed characters, thus causing a contamination problem in many cases.
Secondly, the edges of the inscribed characters are often burred through the exposure to the laser beam. Thus, a deburring processing step needs to be carried out.
FIG. 3 schematically illustrates a cross section of a sintered wafer 1 on which characters have been marked by the conventional laser marking method. This cross-sectional view is drawn after a scanning electron microscope (SEM) photograph has actually been taken. As shown in FIG. 3, a deep concave portion 30 is formed on the surface of the wafer 1 as a result of the laser beam exposure. As measured from the back surface of the wafer 1 in the direction indicated by the arrow a in FIG. 3, the concave portion 30 has a depth of about 30 μm to about 50 μm. A convex portion (or burr) 31 is also formed around the edge of the concave portion. As also measured from the back surface of the wafer 1 in the direction indicated by the arrow b, the burr 31 has a height of several μm. The concave portion may have a width of about 20 μm, for example.
As shown in FIG. 3, a huge number of particles 32 are deposited on the inner surface of the deep concave portion 30 that has been formed as a result of the laser beam exposure. Strictly speaking, some of those particles 32 may have such irregular shapes that cannot be classified among “particles”. However, those irregular ones will also be referred to herein as “particles” for the sake of simplicity. To remove those particles 32 from the wafer 1, a cleaning process such as an ultrasonic cleaning process must be carried out for a long time after the marking process is finished. Even so, it has still been difficult to remove most of the particles 32 that have reached the depth of the concave portion 30.
If a huge number of particles 32 are created during the marking process, some of those particles may be dispersed in the cleaning liquid and then deposited on the other side (i.e., the front surface 2) of the wafer 1 that has not been exposed to the laser beam. In that case, when an insulating thin film of amorphous aluminum oxide, for example, is deposited on the front surface 2 of the wafer 1 with the re-deposited particles 32, then those particles 32 might be introduced into the insulating film. Also, the surface of such an insulating thin film is normally planarized before a magnetic thin film is deposited thereon. Accordingly, if the insulating thin film includes the particles 32, portions of the insulating thin film may peel off locally along with the particles 32 to possibly create pinholes in the insulating thin film during the planarizing process. Also, even if no such pinholes have been created, a portion of the insulating thin film may have its thickness decreased significantly by the particles 32. Then, that portion of the insulating thin film may exhibit decreased insulating properties. Furthermore, even when no such particles enter the insulating film, the marked portion of the back surface of the wafer 1 may still be a dust source. Then, the yield may decrease in a number of subsequent manufacturing process steps, and the quality of the final product itself may deteriorate.
Accordingly, to increase the production yield of thin-film magnetic heads, the insulating film to be deposited on the sintered wafer 1 preferably is as high quality as possible. For that purpose, the conditions for the marking process are preferably controlled so as to eliminate the dust or contamination problem. In addition, once completed as a product, the magnetic head needs to be used in a clean environment. Thus, the presence of any dust would also be a problem that affects normal operation of the magnetic head.
Meanwhile, methods of writing ID information on a sintered wafer by a chemical etching process have also been proposed as replacements for the laser marking process. For example, the applicant of the present application disclosed a technique of forming a shallow concave portion by a chemical etching process while increasing the contrast in Japanese Laid-Open Publication No. 2001-334753. According to this technique, a compound sintered body, made of at least two types of powder particles with mutually different etch susceptibilities, is subjected to a selective etching process such that one of the two types of powder particles is etched preferentially. As a result of such a selective etching process, an unevenness of a very small size, corresponding to the size of the powder particles, is formed on the surface of the compound sintered wafer. Such an unevenness decreases the reflectivity of the wafer surface, thereby creating a difference in contrast between the etched and non-etched portions of the wafer.
The selective etching process disclosed in Japanese Laid-Open Publication No. 2001-334753 can resolve the dust or contamination problem. However, according to this technique, the contrast of the reflected light is not high enough to increase the recognition rate of the ID information sufficiently.